Applicants' invention relates to methods and apparatuses for receiving electromagnetic signals, and more particularly to coherent receivers in digital communication systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM and TIA-136 telecommunication standards and their enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and WCDMA telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the UMTS telecommunication standard. This application focusses on CDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in TDMA and other digital communication systems.
In a CDMA communication system, an information data stream representing, for example, speech to be transmitted, is impressed upon a higher-rate data stream that may be called a “signature sequence” or a “spreading sequence”. Typically, the signature sequence is a binary bit stream that can be replicated by an intended receiver. The information data stream and the signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or −1. This combination of a higher-bit-rate signature sequence with a lower-bit-rate data stream is called “spreading” the lower-bit-rate data stream. It will be understood that information data streams may be encoded for error correction and other reasons and may be combined with other binary sequences that have useful cross—and auto-correlation properties, e.g., Walsh-Hadamard sequences, before the resulting combinations are spread.
A plurality of spread information streams can modulate a radio-frequency (RF) carrier, for example by phase shift keying (PSK), and then be jointly received as a composite signal at a receiver. Each of the spread information signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. The spread information signals can be isolated by correlating the composite signal with the signature sequence, and then the information signals can be reconstituted by decoding appropriately.
Among other things, a coherent receiver in a digital communication system estimates the impulse response of the communication channel through which signals pass from the transmitter to the receiver to optimize its performance. The channel is typically modelled as a tapped delay line, in which the tap locations correspond to signal ray, or path, time delays and the generally complex tap coefficients correspond to channel coefficients. The result of the channel estimation process is a set of estimates of the tap locations and coefficients that is provided to a signal demodulator. The set of estimates is usually obtained through some combination of coherent and non-coherent values that are produced by correlating the received signal with a predetermined training sequence, in TDMA systems, or with a signature sequence, in CDMA systems. For TDMA and other narrowband communication systems, the demodulator is usually a coherent detector, such as a DFE or MLSE equalizer. For CDMA and other wideband communication systems, the demodulator is usually a RAKE receiver.
This sort of receiver is used in many digital communication systems because transmitted signals are reflected by objects, like buildings and mountains, between the transmitter and receiver. Thus, a transmitted signal propagates to the receiver along not one but many paths so that the receiver hears many echoes, or rays, having different and randomly varying delays and amplitudes. In a CDMA communication system, the receiver receives multiple versions of the transmitted composite signal that have propagated along different paths, some of which may have relative time delays of less than one chip. As a result of such time dispersion and in the absence of a RAKE receiver, correlating the received signal with a signature sequence would produce an output having several smaller spikes rather than one large spike. The several spikes are combined in an appropriate way by the RAKE receiver, which is so named because it “rakes” all the multipath contributions together.
FIG. 1 is a block diagram of a typical coherent receiver 1 for a CDMA-type digital communication system. A carrier removal block 100 depicts components used for receiving a modulated carrier signal and extracting the modulation from the carrier, which is a process of signal down-conversion from the spectral region of the carrier to the spectral region of the modulation, usually base band. The modulation, or base-band spread signal, is provided to a delay selector block 101 that determines a set of estimates of tap locations and coefficients, i.e., a set of delays, for the different signal echoes received. This set of estimates is provided to a propagation channel estimator block 102 as described above. The delays can be determined by correlating the received signal with a known portion of the transmitted signal, such as pilot symbols or training sequences, over a pre-defined time window, and selecting from the correlation values based on received power or signal-to-interference ratio (SIR). The pilot or training symbols are used in estimating the impulse response of the propagation channel in block 102 as described.
Many devices and methods for determining and selecting echoes may be used. For example, use of complex delay profiles, which can be averaged to form a power delay profile, is described in U.S. patent application Ser. No. 09/005,580 filed on Jan. 12, 1998, by E. Sourour et al. for “Method and Apparatus for Multipath Delay Estimation for Direct Sequence Spread Spectrum Communication Systems”. Other devices and methods for determining and selecting echoes are described in U.S. patent application Ser. No. 10/017,745 filed on Dec. 14, 2001, by E. Sourour et al. for “Interference Suppression in a Radio Receiver”; and U.S. Provisional Patent Applications No. 60/412,321 filed on Sep. 23, 2002, by X. Wang et al. for “Efficient Multipath Detections” and No. 60/412,899 filed on Sep. 23, 2002, by E. Jonsson for “Objective Multi-path Delay Selection Algorithm for WCDMA”.
The base-band spread signal is also provided to a RAKE block 103 that includes a number of “fingers”, or de-spreaders, that are each assigned to respective ones of the selected echoes. Of course, it will be appreciated that the RAKE block could include one finger that is re-used for each selected echo if the block operates fast enough. Each finger combines an echo selected by block 101 with the spreading sequence so as to de-spread the received composite signal. The RAKE block 103 typically de-spreads both sent information data and pilot or training symbols that are included in the composite signal. It will be appreciated that one or more fingers of the RAKE receiver block can be used in the delay selection block 101 to determine the echo delays that are selected. Such fingers are sometimes called “path searchers”.
Various aspects of RAKE receivers are described in G. Turin, “Introduction to Spread-Spectrum Antimultipath Techniques and Their Application to Urban Digital Radio”, Proc. IEEE, vol. 68, pp. 328–353 (March 1980); U.S. Pat. No. 5,305,349 to Dent for “Quantized Coherent Rake Receiver”; U.S. Patent Application Publication No. 2001/0028677 by Wang et al. for “Apparatus and Methods for Finger Delay Selection in Rake Receivers”; U.S. patent applications Ser. No. 09/165,647 filed on Oct. 2, 1998, by G. Bottomley for “Method and Apparatus for Interference Cancellation in a Rake Receiver” and Ser. No. 09/344,898 filed on Jun. 25, 1999, by Wang et al. for “Multi-Stage Rake Combining Methods and Apparatus”.
In a combiner block 104, de-spread information data is multiplied by the complex conjugates of respective propagation channel estimates to remove distortion introduced by the propagation channel. The products are summed over the selected number of delays and fed to a decoder and de-interleaver block 105, which may include a Viterbi decoder or a Turbo decoder, for example. The block 105 produces decoded symbols that replicate the transmitted information data.
For a digital receiver implemented in one or more integrated circuits, the decoder and de-interleaver can require a large area of silicon, and it is therefore desirable to keep the number of bits used to represent numbers in this block as low as possible. If the delays provided from the RAKE block 103 to the combiner 104 are highly correlated, then the combiner adds, in effect, the same signal twice. This can cause serious loss of soft information due to overflow after truncating the outputs of the combiner due to the limited number of bits in the decoder. Besides the absence of true diversity combining and its benefits, this results in deteriorated decoding performance, particularly when the delay selector 101 chooses multiple delays that all originate from the same path. This is likely when the selector's delay resolution is less than one chip and is due to temporal broadening of the echoes by pulse shaping of the transmitted signal.